Progesterone receptor and the hsp90 chaperone pathway. When in its inactive form in cytosol extracts, the avian progesterone receptor (PR), like most steroid receptors and several other cell signaling proteins, is complexed with the heat shock protein hsp90. This complex does not form through a simple association, but through a process that requires ATP hydrolysis and several additional proteins. Other proteins involved in receptor complex formation are hsp70, three hsp70 co-chaperones, hsp40, Hip and Hop, four hsp90-binding TPR proteins and p23. We have developed a chemically defined system with purified proteins that has the capacity to assemble PR complexes with hsp90. Only five of the above proteins are needed for this in vitro assembly process which appears to proceed through three steps to generate PR with hormone binding activity. The objective of the proposed studies is to further our understanding on the assembly of receptor complexes, their dissociation upon hormone binding, and the functional significance of the receptor-associated proteins. This will be accomplished by extensive use of the in vitro defined assembly system. Events at each step of PR complex formation will be characterized in terms of the dynamics of protein interactions and the utilization of ATP. Modifications in the system will be made that allow the study of hormone- dependent dissociation of PR from hsp90. This will probably require the identification of new factors needed for the dissociation process. Finally, we will characterize the domains or elements of the PR that are interaction sites for binding hsp70 and hsp90 and investigate the properties which promote heat shock protein recognition. These studies should lead toward a mechanistic description of the steroid receptor activation process and should provide valuable information for understanding the mechanisms of action of the molecular chaperones hsp70 and hsp90.